New 451 HTTP Code Identifies Censored Websites

by Tracy Knauer • January 1, 2016

Identifying sites that have been blocked due to Internet censorship has become easier thanks to a new HTTP status code, 451. Here are the details.

HTTP status codes are used to classify different types of transactions between Web servers and browsers. In most cases users never see them, although some common codes — like the 404 “page not found” error code — are familiar to many people.

Status code 451 was proposed a couple of years ago as a way to indicate that Web content has been “blocked for legal reasons.” Last week, the Internet Engineering Steering Group approved the proposal, making 451 an official HTTP status code.

Web servers could previously deliver 403 codes, which indicated that they were denying access to a page. However, a 403 code does not necessarily mean the content is censored in a certain location. It could be blocked to everyone for security reasons, for instance.

With the approval of the 451 code, it is now easier for websites to identify content that is specifically censored. Companies like Twitter can use the code to indicate to users that certain pages have been blocked by government authorities in a particular country, even though they are available elsewhere.

With that information, users would know that they could use a VPN or Tor to access the censored content.

Code 451 — which, by the way, is a reference to Ray Bradbury’s novel Fahrenheit 451 — is also useful because it makes it easier to identify censored websites in a systematic way. Software can now automatically crawl websites and collect 451 codes in order to build databases of censored content.

The only big caveat is that “you can’t guarantee that all attempts to censor content will be conveniently labeled by the censor,” as one of the backers of the proposal explained in a recent blog post. Either website administrators or the authorities blocking sites at the firewall level have to choose to use the code.

That means that the absence of a 451 code does not necessarily mean content is not being censored, which is important to keep in mind.